1. Field of the Invention
This invention relates to communication systems. In particular, the invention relates to power amplifiers.
2. Description of Related Art
An important component in a communication or signal processing system is the power amplifier (PA). A PA usually consumes a large amount of power to provide power gain to a transmitted signal. A major problem in radio frequency (RF) communication systems is the non-linearities introduced by the operation of the PA in the signal processing chain.
The non-linearity of the PA results in distortion to the transmitted signal. Higher class PA's achieve theoretically better efficiency, expressed as the ratio of the generated output radio frequency (RF) power to the consumed direct current (DC) power, but introduce significant non-linearities to the transmitted signal. The distortion is normally in the form of severe compression which distorts the amplitude of the signal. In the extreme case, the compression behaves like a hard limiter which truncates the signal at a certain threshold.
Although constant envelope waveforms possess some inherent immunity to the compression effects caused by the PA's, they still suffer from spurious consequences. In addition, amplitude modulation (AM) to phase modulation (PM) conversion is another side-effect of these PA's in which the phase modulated signals are greatly distorted. For non-constant envelope modulation schemes, such as AM, the amplitude distortion due to the PA clearly produces undesirable results.
There exist a few techniques to reduce the non-linearity effect of the PA's. One such technique is to design the PA with selective components and carefully designed internal compensating circuits. For example, balanced amplifier circuits can be combined to achieve high powers. Internally matched structures can be included to reduce parasitic effects. However, this technique increases the circuit complexity, results in expensive or bulky PA's which may not be suitable for wireless applications, and provides marginal improvement at best. Another technique is to compensate the non-linearity by using a separate filtering system which has phase and amplitude characteristics complementary to the PA characteristics. A disadvantage of this technique is the difficulty in matching both the phase and amplitude of the filter characteristics to the PA characteristics. Another disadvantage is the inability to respond to the system dynamically as the frequency or phase of the signal changes, or the characteristics of the PA or filter change (independently of one another).
Therefore there is a need in the technology to provide a simple and efficient method to linearize the characteristics of the PA without compromising power efficiency.